Add test dependencies

Author: amacneil

composer.json

@@ -25,6 +25,12 @@
     "autoload": {
         "psr-0": { "Omnipay\\Tests\\" : "src/" }
     },
+    "require": {
+        "guzzle/plugin-mock": "~3.1",
+        "mockery/mockery": "~0.8",
+        "phpunit/phpunit": "~3.7.0",
+        "squizlabs/php_codesniffer": "~1.4"
+    },
     "extra": {
         "branch-alias": {
             "dev-master": "2.0.x-dev"